Despite Construct being mostly 2D, 3D models can still be handy. Animating a 3d model and rendering the results is usually easier than drawing the animation frame by frame. Unless you use something like Moho (now Anime Studio), that is.
Anyway, let's make a list of free 3d resources.
Here's one to get things going:
http://free3dmodels.fleshsoft.eu/ - a bunch of free Blender models.
Also, don't forget http://turbosquid.com/, it has quite a lot of free models too. Just set the maximum price to 0 in the search.